Devices for holding the fuel rods in the cells of spacer grids have therefore been proposed which provide an exceptional compromise between these seemingly contradictory requirements. For making the structural grids and the elements for holding the fuel rods, it is furthermore necessary to use materials which have satisfactory mechanical properties at the operating temperature of the nuclear reactor and have sufficient stability under the conditions prevailing in the nuclear reactor core, in the presence of the cooling fluid of the reactor and the irradiation.
It has, for example, been proposed to make the spacer grids of an alloy with high yield strength, such as a nickel alloy, springs and rigid stops for holding the rods being obtained by cutting and pushing in some portions of the straps constituting the grid. Such a solution has the drawback of introducing a relatively large volume of neutron-absorbing material into the assembly.
It has also been proposed to make the straps constituting the cells of the grid from a material which is a weak neutron absorber, such as a zirconium alloy, the elements for holding the fuel rods being attached to these zirconium alloy straps. The holding elements may consist of attached springs made of a highly elastic alloy and of rigid stops formed on the straps constituting the various walls of the cells of the grid, in such a way that in each cell the springs are on walls opposite the walls on which the rigid stops are formed. The springs may consist, for example, of a leaf of elastic material, at least partially enclosing the strap and closed on itself.
FR-A-2,474,229 has proposed the use of double springs, i.e., springs including two active parts which are intended to come into contact with a fuel rod, in two cells located on either side of the strap on to which the spring is fixed, on the majority of the walls equipped with springs, and single springs including a single active part on the other walls equipped with springs.
The spacer grids consist of interlaced zirconium alloy straps defining the regular lattice of cells into which the fuel rods are introduced. The cells defined by the straps are of generally parallelepipedal shape with square cross-section and have mutually parallel corner edges, each consisting of the intersection of two straps forming a cross-brace consisting of four dihedra, the angle of which is generally 90.degree., having in common one corner edge common to four cells of the grid.
In some types of fuel assembly, the spacer grids are all identical and consist of interlaced zirconium alloy straps which are assembled with each other, on to which springs are attached.
Fuel assemblies which include a plurality of types of spacer grids having different functions are also known.
In particular, a fuel assembly is known which includes at each of its ends a rod-holding grid made entirely of martensitic steel, intermediate grids, between the end grids, constituting structural grids which include interlaced zirconium alloy straps and attached springs and, interposed between the structural grids, in the upper part of the fuel assembly, grids for mixing the cooling fluid of the nuclear reactor.
In the case of fuel assemblies in which all the grids are zirconium alloy structural grids including attached springs, the springs are the only holding elements which are interposed, in each of the cells, between the rod housed in the cell and that wall of the grid on to which the spring is fixed. If the fuel rod is subjected to a stress tending to displace it towards a cell wall on which a spring is mounted, the spring can be formed by compression so that the rod moves inside the cell in a transverse direction.
In the case of holding grids which are different from the structural grids, the springs bear on the rods and also risk being deformed by external forces exerted on the fuel assembly.
In the case of the peripheral cells of the grid and, in particular, in the case of the corner cells of the grid, the fuel rods may undergo large displacements due, for example, to an impact or the fuel assembly catching on a second fuel assembly or on an obstacle, during handling of the assembly.
This may result in damage to the spacer grid of the assembly and risks of the grids of the assemblies catching when they are handled.
In the case of the internal cells of a structural grid, the springs attached to the walls of the cell may also be subjected to large forces during the handling of the fuel assembly or during operation of the nuclear reactor.
WO-A-9205566 and FR-A2,168,059 describe a spacer grid for a fuel assembly of a nuclear reactor cooled by boiling water, consisting of tubular walls rigidly assembled together in juxtaposed positions. Fuel-rod holding springs are placed so as to surround opposite parts of two adjacent walls. The opposite parts of the adjacent walls are pushed in so as to form dimples, in the shape of a spherical cap, facing the branches of the spring.
In a fuel assembly for a boiling water reactor which includes such grids having juxtaposed tubular walls, the mechanical behavior of the fuel rods and the springs supported by the walls is essentially different from the mechanical behavior of the rods in a fuel assembly for a nuclear reactor cooled by pressurized water, in which the grids consist of plane straps.
The solutions proposed for the problems of spring deformation and rod displacement cannot therefore be adapted from the case of fuel assemblies for a boiling water reactor to the case of pressurized water reactors.
In the case of the structural grids, the straps, generally made of zirconium alloy, are engaged in pairs at their intersections, by means of an assembly slot; these plates must furthermore be welded in order to make the spacer grid sufficiently rigid and solid.
Because the straps are thin, some precautions must be taken during welding in order to avoid deforming or oxidizing them. Efforts are therefore made to limit the heating of the straps during the welding and to distribute as much as possible the thermal stresses due to the welding of the grid.
For example, EP-A-0,088,021 proposes laser welding of the straps. Each of the cross-braces consisting of the intersection of two straps is welded in two opposite dihedral corners of the cross-brace and only in these two corners. The cross-braces are welded successively, in a defined sequence, by moving the assembled grid, which is held by a mounting frame, under a welding installation.
The drawback of such a fastening method is that the straps are assembled at each of the cross-braces only by a welding line generally lying in a bisector plane of two opposite dihedra. This may result in a lack of rigidity and slight defects in the positioning of the straps relative to each other.
Spacer grids with attached springs may also include other defects inherent in the construction and the design of the spacer grid, when mounting the springs, when producing the bearing dimples for the fuel rods, when mounting the guide tubes or else when making the mixing or guide vanes by cutting the edges of the straps or the belt surrounding the spacer grid.